Bulb-type fluorescent lamps have been studied as a new type of illuminating device that can be applied directly to conventional incandescent lamp sockets and use less electricity, while maintaining brightness at the conventional level. However, since bulb-type fluorescent lamps are used in the same space as the space used for conventional incandescent lamps, the size of the lamps is limited. Also, fluorescent tube is required to be as long as possible so as to increase brightness. Therefore, bulb-type fluorescent lamps which have lengthened fluorescent tubes have been proposed. Such lamps include one using a double U-tube, which is formed by bending a straight fluorescent tube into a "U" shape and again bending the tube into a U shape in a direction at right angles to the previously bent surface; a lamp formed by binding a plurality (from four to six) of straight tubes with a bridge; and a lamp disclosed in U.S. Pat. No. 4,772,819 in which a plurality of ring fluorescent tubes are connected by a bridge.
In the conventional bulb-type fluorescent lamp, one fluorescent tube is bent into a U shape twice so as to form a double U-tube. Thus, although the method of forming the tube is relatively simple, limited space is not sufficiently utilized, and the amount of dead space is large. On the other hand, the conventional lamp which is formed by connecting a plurality of fluorescent tubes by a bridge requires a connecting process, so that the manufacturing process is not efficient. Also, the part above or below the bridge does not contribute to light generation, which creates unused or "dead" space.